Fibrotic diseases such as scleroderma, severe chronic asthma, pulmonary fibrosis, and cardiac fibrosis kill tens of thousands of people each year in the US alone. Growing evidence suggests that in a fibrotic lesion a subset of blood monocytes enters the tissue and differentiates into fibroblast-like cells called fibrocytes, causing tissue dysfunction. We found that a blood protein called serum amyloid P (SAP) inhibits this differentiation in vitro. Injections of SAP prevent bleomycin-induced lung fibrosis in rats and mice, and ischemia-reperfusion induced cardiac fibrosis in mice. Some children with pulmonary fibrosis have low serum levels of SAP, and an exciting possibility is that SAP might be useful as an anti-fibrotic in humans. A critical need is to be able to identify fibrocyte precursors, early fibrocytes, and mature fibrocytes, so that in the early stages of treating a patient one could take a biopsy and determine if the treatment is affecting fibrocyte differentiation. Although there are several antigens that serve as markers for mature fibrocytes, little is known about the phenotype of early fibrocytes and the identity of the monocyte subset is unknown. We propose three specific aims to elucidate fibrocyte differentiation. First, we will determine the time course of appearance of known fibrocyte markers during human, rat, and mouse fibrocyte differentiation in vitro, to identify the earliest possible markers. We will then use this information to determine the stage at which SAP inhibits fibrocyte differentiation. Second, we will determine whether just one or several known subsets of monocytes can differentiate into fibrocytes, and thus whether known markers can serve as biomarkers of fibrocyte precursors. Third, we will induce pulmonary fibrosis in mice, and determine the timecourse of marker appearance in vivo, and use this information to test the hypothesis that SAP inhibits fibrocyte differentiation to inhibit fibrosis. Lay Language: We anticipate that identification of the markers for the circulating fibrocyte progenitor cells might lead to diagnostic assays for patients at risk for fibrosing diseases, determining what markers are expressed by fibrocytes at different times during their development will allow one to monitor clinical efficacy early in a clinical trial of an anti-fibrotic drug, and that elucidating at what stage SAP inhibits fibrocyte differentiation in a tissue will help us understand how we might use SAP to regulate fibrosis in patients.